Methods of forming a direct positive image, which involve using unprefogged internal latent-image type silver halide photographic emulsions, and subjecting them to surface development subsequent to or simultaneous with a fogging process performed after imagewise exposure, are well known.
The term "internal latent-image type silver halide photographic emulsions" as used above refers to silver halide photographic emulsions of the type which have light-sensitive nuclei mainly inside the silver halide grains, and form a latent image predominantly inside the grains by exposure.
Various methods are known in this field, and the principal ones are described, e.g., in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, British Patents 1,151,363, 1,150,533 and 1,011,062.
These known methods can generally provide photographic light-sensitive materials having a comparatively high photographic speed, considering that they are direct positive type.
Details of the mechanisms of the direct-positive image formation are described, e.g., in T.H. James, The Theory of the Photographic Process, (4th ed.), chap. 7, pp. 182-193, and U.S. Pat. No. 3,761,276.
Specifically, it is believed that the surface desensitization attributable to the internal latent image produced inside silver halide grains by the first imagewise exposure brings about selective formation of fogged nuclei at only the individual surfaces of silver halide grains present in unexposed areas, and a conventional surface-development processing subsequent to the imagewise exposure produces a photographic image (direct-positive image) in the unexposed areas.
As the means of selectively forming fogged nuclei, as described above, there are known a method of giving a second exposure to the whole surface of a light-sensitive layer, which is generally called "an optical fogging method" (as described, e.g., in British Patent 1,151,363), and a method using a nucleating agent, which is called "a chemical fogging method". Details of the latter method are described, e.g., in Research Disclosure, vol. 151, No. 15162, pp. 76-78 (Nov., 1976).
Formation of direct-positive (color) images can be achieved by subjecting silver halide photographic materials of the internal latent-image type to a surface color development-processing after or as they undergo a fogging treatment, and then (to a bleach processing and) a fixation processing successively (or a bleach-fix processing). After (bleach and) fixation processings, washing and/or stabilization is performed. (In parentheses processes for obtaining color images are shown.)
As for the nucleating agent used in the foregoing "chemical fogging method", hydrazine compounds are well known.
The nucleating agents of hydrazine type, though superior in discrimination because they generally cause a great difference between the maximum density and the minimum density, have the disadvantage that they require a high pH condition (pH 12) in the development-processing.
As for the nucleating agents which can function under a lower pH processing condition (pH.ltoreq.12), heterocyclic quaternary ammonium salts are known, and described, e.g., in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Patent 1,283,835, JP-A-52-3426 and JP-A-52-69613 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). In particular, propargylor butinyl-substituted heterocyclic quaternary ammonium salts disclosed in U.S. Pat. No. 4,115,122 are excellent nucleating agents in respect of discrimination in direct positive silver halide emulsions. However, they are unsatisfactory because, e.g., when sensitizing dyes are added to the foregoing silver halide emulsions for the purpose of spectral sensitization, competitive adsorption to silver halide emulsion grains occurs between the sensitizing dyes and the nucleating agents of heterocyclic quaternary ammonium salts. This requires the addition of a large quantity of quaternary salt type nucleating agent because of its weak adsorptivity, to cause uneven density and loss of color balance, particularly in the case of multilayer color photographic materials. Undesirable influences of this phenomenon tend to become more serious under running processing or upon storage under high temperature and high humidity conditions.
With the intention of solving the foregoing problem, U.S. Pat. No. 4,471,044 discloses a quaternary salt type nucleating agent which contains a thioamide group as a group for accelerating the adsorption to silver halide grains. Though introduction of the adsorption accelerating group can reduce the addition amount of the nucleating agent required for achievement of sufficiently high Dmax and reduces the a decrease in Dmax upon storage under high temperatures, the effect is not yet satisfactory.